Fuses and Disconnects


The proper selection of fuses and other protection techniques can greatly affect the reliability of SCR controllers.

Some SCR controllers are provided with fuses to protect the SCR's, some controllers have electronic fusing, and low cost controllers are provided with no SCR fusing.

The type of fuse depends, of course, on the load and the local and national codes.

In general a fast acting fuse such as the JJS or JJN fuses provided by Littelfuse have proven to be a good compromise for reliability, cost and protection.

  • Class T fuses are typically less expensive and more readily available than semiconductor fuses.
  • Class T fuses are "listed" for branch service protection, while semiconductor fuses are not.
  • Semiconductor fuses have the tendency to fatigue fail, resulting in nuisance shutdown

The vendor of the controller should be able to recommend the type of fuses required for any specific application.

It is important to note that fuses provided with the controller are intended to protect the SCR's and may not be the correct fuse for load protection.


SCR's have a small leakage current in the off state. Therefore, it is imperative that a mechanical disconnect be used to completely remove power prior to servicing.

Controllers with over-current trip can be configured to operate shunt trip or under voltage trip circuit breakers to remove power in the event a load fault occurs.

Visit our FAQ for more information about fuses and disconnects.